The purpose of an intake control is to adjust the air intake flow to the engine air demand to achieve an optimum performance with respect to one another whereby to control the entire system as one compatible unit. The air intake has the purpose of feeding the required amount of air at minimum loss and in a properly homogeneous condition to the engine. Flow losses result, for example, from friction, compression shocks and excessive flow resistances; air inhomogenities result, for example, from pressure irregularities. If both disadvantages can be substantially avoided, then the power system of a given aircraft will produce optimum thrust. Furthermore, it is desirable that the air intake and the engine are in continuous cooperation or otherwise performance losses will occur, the consequences of which can be of various kinds. First of all, if the intake and engine operation go out of proper relationship to each other, compression vibrations occur in the power system, which, if they exceed certain permissible limits, can result in the mechanical destruction of air intake and/or engine.
The above-discussed relationships are of a special importance for power systems which are intended to operate through a larger range of flight speeds. Such engines have widely varying range of air demand depending on the respective flight conditions, which among other things is determined by mach number and engine speed. High-powered engines require, for example during starting an extremely high "corrected rate of airflow," which means that the most narrow flow cross section of the intake must be dimensioned as large as possible. Compared with this, the "corrected rate of airflow" must be extremely low at high supersonic flight, for example at mach 2.2, wherein the most narrow flow cross section of the intake must be dimensioned as small as possible to achieve the desired configuration of the compression shocks in the inlet area of the air intake.
Since each air intake of fixed cross section has a stable working range of only limited extent, an adjustable air intake must be provided, particularly for high-powered engines, in order to provide for appropriate adjustment. The present generally common automatic control or regulation of the air intake is taken over by the so-called intake control system.
For example a two-dimensional air intake for a high-powered aircraft has been suggested, which has a total of three ramps over its length and the longitudinal axis of which -- as is common -- extends parallel to the x-axis of the aircraft. Starting from the front of the aircraft, the first ramp is constructed fixedly and the second and third ramps are movable to vary the air intake. For this purpose the second and third ramps are connected mechanically through jointed means and linkages. An electrohydraulic operating cylinder engages the linkage and such cylinder is capable of adjusting synchronously the two movable ramps. The intake control system automatically controls the operating cylinder or the ramp actuator and said intake control system is, in the latest designs, based on an electronic digital calculator. A relatively wide gap exists between the second and the third ramps of the aforediscussed air intake, through which gap flows outwardly a part of the inner boundary layer and a part of the intake air. The space above the second or the center ramp is used as the measuring chamber for a pressure which serves as control parameter for the known intake control system. Furthermore, the following values are considered in the intake control: the static and the total pressure of the external flow and the angle of attack of a given aircraft. The relationship of the static (barometric) pressure to the total (pitot tube) pressure of the external flow indicates the flight mach number.
It is furthermore known to use pressure ratios in the air intake of jet engines for controlling the air intake. It is, for example, known to regulate the intake geometry as a function of the mach number and a typical pressure ratio (DAS 1,202,267 and U.S. Pat. No. 3,181,818). Another known system of this type, in which the main control parameter is the position of the compression shocks, uses pressure measuring points in the front part of the intake and a second variable element in the form of a blow-off valve (French Pat. No. 2,026,964). In all of these systems it is not possible in a simple manner to effect a required change of the control parameter, as same might become necessary, in consideration of the output and stability of the controlled system.
The basic purpose of the invention is to produce an intake control of the above-mentioned type, which permits modification of the control parameter in a selected manner so that an intake control for optimum performance is achieved in the entire flight speed range, wherein the required stability of the aerodynamically closed control loop and simultaneously the compatability between the intake and engine are assured. It is also the purpose of the invention to eliminate the bypass door-like structure used in combination with the structure which varies the geometry of the air intake duct.
According to the invention this purpose is attained by using a variable dimensionless relationship of the pressure above a movable center ramp to the total pressure of the external flow as control parameter. In particular, the invention is characterized by calculating this dimensionless pressure ratio relative to an operating cylinder (actual position) feedback signal and utilizing a sum of the ratio and other computed values as a determining value to adjust the optimum performance point of the power system, which latter has been previously determined through wind tunnel and/or flight testing.